Problem: The scale on a map is 3cm : 9km. If two cities are 30cm apart on the map, what is the actual distance in km between the two cities?
Solution: The scale means that every 3cm on the map corresponds to an actual distance of 9km. A distance of 30cm on the map is the same as 10 $\cdot$ 3cm. The actual distance between the two cities is 10 $\cdot$ 9km, or 90km.